About 60 people get measles in the United States each year on average, the CDC says. Since measles stopped circulating in this country in 2000, the highest number of cases occurred in 2008, when 140 Americans got measles, and 2011, when 220 cases were reported.

The CDC is worried because measles is highly contagious and can be life-threatening. So far no one has died from the measles this year, but 17 people were hospitalized, according to the CDC.

"The increase in measles cases in the United States in 2013 serves as a reminder that imported measles cases can result in large outbreaks, particularly if introduced into areas with pockets of unvaccinated persons," researchers wrote.

All of the outbreaks were sparked by someone who was infected in another country, usually somewhere in Europe, brought the virus to the United States and exposed people who hadn't been vaccinated.

"In some communities, people have been rejecting opportunities to be vaccinated," Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters during a briefing.

The largest outbreak, which involved at least 58 people, occurred in March among Orthodox Jews in Brooklyn, N.Y., after an unvaccinated teenager infected with measles returned from visiting Britain. That was the largest measles outbreak in the United States since 1996.

The second-largest outbreak, which involved at least 23 cases, occurred in April in North Carolina, mostly among people who had not been vaccinated because of religious objections, the CDC said. That apparently started after someone returned from a three-month visit to India.

Overall, 18 of the cases that have occurred so far this year were among children younger than a year old, while 40 people were between the ages of 1 and 4, 58 were among those ages 5 to 19, and 43 were 20 or older, according to the CDC.